Section 68. A town which has acquired a municipal lighting plant shall not sell it for the purpose of abandoning the distribution of gas or electricity to its inhabitants until such sale has been authorized in the manner and by the votes prescribed for the acquisition of such plants by sections thirty-five and thirty-six. No sale of such a plant shall be made for any purpose until the department, after notice and a public hearing, has determined that the facilities for furnishing and distributing gas and electricity in the territory served by such plant will not thereby be diminished, and that such sale and the terms thereof are consistent with the public interest.